Many computer systems process and/or produce large volumes of data. In order to maintain such data persistently, computer systems typically store the data on data storage media, such as magnetic disks or tapes. It is common for a computer system to use a device known as a drive to access data on a unit of data storage media, by causing the drive to either read data from or write data to the data storage media. Drives usually contain an accessing component, also called a "head," for reading data from or writing data to a unit of data storage media; a positioning component for positioning the unit of data storage media relative to the head; and control logic for controlling the head and positioning component in order to read from or write to the unit of data storage media.
Unlike magnetic disks that are sealed inside their drives, tape cartridges and units of other storage media are separable from their drives. This allows multiple tape cartridges to be accessed by the same tape drive. Indeed, some computer systems use large tape subsystems that combine a number of tape drives with a large number of tape cartridges. Many such tape subsystems use robotic mounting systems that transfer tape cartridges between tape drives and storage locations.
The fact that multiple tape cartridges may be accessed by the same tape drive adds a degree of flexibility to decisions to acquire additional tape capacity. That is, two different kinds of tape capacity may be maintained independently. By acquiring additional tape cartridges, one can increase the total storage volume of tape cartridges without increasing the total number of tape drives. Similarly, by acquiring additional tape drives, one can increase the number of tape cartridges that may be accessed simultaneously without increasing the total number of tape cartridges.
For magnetic disks that are sealed into their drives, the decision to acquire additional drives is very often based on a need to increase total storage capacity. The decision to acquire additional tape drives, however, is independent of total storage capacity needs. Instead, the decision to acquire additional tape drives is best based on the number of tape cartridges that need to be simultaneously accessed by the computer system.
In some cases, automated systems for analyzing demand in a tape subsystem utilize a "peak mounts" time-indexed statistic indicating the largest number of tape drives simultaneously accessing tape cartridges during each hour. This approach is sometimes described as the "concurrent allocation" approach to demand analysis. This peak mounts statistic is very reflective of instantaneous demand, but is a poor indicator of overall, time-distributed demand. The peak mounts statistic is therefore a disadvantageous measure of demand in computer systems that execute jobs having some tolerance for delays in performing tape cartridge access requests, where overall demand is more important than instantaneous demand. An automated method for providing a more useful measure of drive utilization would therefore have significant utility.